أَمَرَنِي رَسُولُ اللَّهِ صلى الله عليه وسلم بِنَوْمٍ عَلَى وِتْرٍ وَالْغُسْلِ يَوْمَ الْجُمُعَةِ وَصِيَامِ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ .
It was narrated that Abu Hurairah said: "The Messenger of Allah commanded me to sleep after praying Witr, to perform Ghusl on Friday and to fast three days of each month." (Sahih).